Satellite Positioning Receiver and Proxy Location System

ABSTRACT

To reduce power consumption in a user terminal, especially mobile devices, a system and method are introduced that use terrestrial beacons as a location proxy when satellite positioning signals are not available. The geographic locations of the terrestrial beacons need not be known to use the beacons as a proxy for a satellite positioning signals derived location.

CROSS-REFERENCE

This application is a continuation of U.S. application Ser. No.12/641,807, filed Dec. 18, 2009, currently pending, which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a method and apparatus for satellitepositioning systems generally and, more particularly, but notexclusively, to a multi-receiver approach to allow location in-buildingor whenever or wherever sufficient satellite positioning signals are notavailable.

BACKGROUND

Positioning systems, such as Global Navigation Satellite Systems (GNSS)(the current operational example is the US NavStar GPS system)receivers, are increasingly being integrated into battery-operatedterminals (i.e., netbook personal computers and mobile wirelessdevices). The orbiting satellite constellations provide low-power,wideband radio signals that the receiver uses to estimate the locationof the user terminal. These location estimates are typically provided toon-board applications running on a co-located processing unit. Tomaximize the duration between battery recharge, fuel cell fill-up, orreplacement of the power source, power consumption by the locationreceiver subsystem should be limited. Currently location receiversubsystems are only powered when actively required by a location-basedapplication and even then are periodically powered down resulting in anperiodic but intermittent set of location estimates.

A variety of terrestrial digital radio beacons with known signalcharacteristics (transmission frequency, frequency band, demodulation,framing, framing rate, bit patterns) and broadcast identifiers areavailable in the form of wireless local area networks (WLAN) and widearea cellular communications networks (Wireless Communications Networks(WCNs) are deployed to allow users to access said networks.

SUMMARY

The present invention concerns a method and apparatus to reduce powerconsumption in a user terminal, especially mobile devices, by usingterrestrial beacons as a location proxy when satellite positioningsignals are not available. The geographic location of the terrestrialbeacons need not be known to use the beacon as a proxy for a satellitepositioning signals derived location.

For example, in an illustrative embodiment, a mobile device comprises aGNSS receiver configured to receive GNSS signals; a second receiverconfigured to receive a terrestrial beacon signal; and a controller. Inthe illustrative embodiment, the controller is configured to detect whenGNSS signals are unavailable due to signal blockage and, in responsethereto, to use a received terrestrial beacon signal to determinewhether a previous GNSS location update is valid. Other aspects of thedisclosed subject matter are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary as well as the following detailed description arebetter understood when read in conjunction with the appended drawings.For the purpose of illustrating the invention, there is shown in thedrawings exemplary constructions of the invention; however, theinvention is not limited to the specific methods and instrumentalitiesdisclosed. In the drawings:

FIG. 1 illustrates the use of a local terrestrial beacon as a proxyreplacement for satellite signaling.

FIG. 2 a depicts the events in the use of a terrestrial beacon as aproxy replacement for satellite signaling.

FIG. 2 b depicts the events in the discontinuing use of a terrestrialbeacon as a proxy replacement for satellite signaling.

FIG. 2 c depicts the events in the failure case in discontinuing use ofa terrestrial beacon as a proxy replacement for satellite signaling.

FIG. 3 depicts a multi-receiver location-capable subsystem for a mobileor portable device

FIG. 4 illustrates the use of timing from a terrestrial wirelesscommunications system as a proxy replacement for satellite signaling

FIG. 5 illustrates the use of received power from a terrestrial wirelesscommunications system as a proxy replacement for satellite signaling

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

We will now describe illustrative embodiments of the present invention.First, we provide a detailed overview of the problem and then a moredetailed description of our solutions.

The Self-positioning receiver (also known as a Location Device Platform(LDP)) is a multi-receiver device used to create and deliver locationestimation on a periodic or ad hoc basis to a location application. TheLDP is assumed to be battery powered and thus have a limited usefullife, so techniques to limit power consumption are vital to the durationof service provided. The LDP circuitry is designed to be part of agreater device such as a mobile phone, personal or auto navigationsystem, or location-aware tag. The device need not have transmissionfacilities since the proxy technique does not require duplexcommunications with a wireless network.

When GNSS signals are unavailable due to signal blockage, as determinedby a failed GNSS location update, the LDP can use its secondaryreceiver(s) received network parameters such as beacon power, beaconidentifiers, and/or signal timing advance or round-trip-time, todetermine if the last known GNSS derived position is still valid. Bysuspending the GNSS searches for an interval or until a specific eventis met, the GNSS related circuitry and processing can be poweredcompletely down, extending battery life.

A. The LDP

Multi-Receiver Devices

The LDP is a multi-receiver device. The GNSS receiver is nominally usedfor location estimation. A proxy receiver (which may be part of atransceiver) is used for collection of signal information from aterrestrial beacon. A Software Defined Radio (SDR) can be used toemulate the multiple receivers and sample the GNSS and terrestrialnetworks. For increased performance, more than 2 receivers can be usedin parallel to allow concurrent GNSS and multiple terrestrial beaconsampling.

Beacons

Beacons are radio transmitters using established frequencies orfrequency bands that transmit data which includes identifyinginformation according to a pre-established scheme. Use of digital databroadcasts by a beacon allows for identification of the beacon and itsnetwork. The network timing is developed from the frames transmitted bythe beacon (the pilot channel or synchronization channel). Examples ofbeacon transmissions include the Broadcast Control Channel of cellularnetworks and the Beacon Frame of an IEEE 802.11 network, and examples ofbeacon identifiers include the Cell-ID in CDMA and UMTS cellularnetworks, the Cell-Global-Identifier (CGI) in GSM cellular networks,Base Station Identifier (BSSID) in IEEE 802.11 wireless local areanetworks.

Passive Beacon Power Levels and Proximity

The LDP can passively use a beacon as a proxy for a high accuracy GNSSlocation estimate by measuring the signal strength of neighboringbeacons during the periodic sampling event. Since both the GNSS receiverand the terrestrial beacon receiver collect information concurrently, ameasurement of the proxy beacon's transmit power can be taken andcompared to the beacon's transmit power recorded at the last GNSSlocation fix. A set of thresholds set using the recorded beacon power isestablished by the LDP so that motion toward or away from the proxybeacon can be detected on subsequent sampling periods.

FIG. 1 illustrates the use of a local terrestrial beacon (in thisexample, a short range Wireless local area network (WLAN) Access Pointsuch as a WiFi (IEEE 802.11) node) as a proxy replacement for satellitesignaling. In FIG. 1 the mobile device 101 is shown outside a building107 with the GNSS satellite constellation 108 with unimpaired radiobroadcasts 111 to allow determination of location. In this example, asingle satellite 109 of the constellation 108 is shown with thenavigation radio broadcast signal 110 blocked by the building 107.Within the building is a WLAN node 105 with broadcast signal 106 whichcan be detected by the mobile device 101 receiver outside the building107.

The same mobile device 101 is shown after moving 103 into a location 104inside the building 107. While the GNSS satellites 108 broadcast signals111 109 are now blocked by the building, the mobile device 101 can stilldetect the same broadcast 109 from the WLAN node 108. The individualmembers of the GNSS satellite constellation 108 with broadcast signals111 or 110 may still be detected, but less than 4 satellite broadcastsare insufficient for calculation of a GNSS-based location estimate.However, since the identifiable local broadcast signal 106 can still bedetected the last GNSS developed position estimate can be consideredvalid as long as the proxy beacon transmission 106 can be detected.

FIG. 2 a details the operative steps in use of a terrestrial beacon as aproxy to determine the validity of the present position versus aprevious GNSS acquired position.

The GNSS receiver subsystem must develop a first valid GNSS position201. This position includes latitude, longitude, altitude, timing, andoptionally velocity. Closely parallel and nearly simultaneously, theBeacon Receiver Subsystem detects and identifies radio beacons inpredefined transmission bands 202. Information on the beacons includestransmitter identification and can include received power, receivertiming, or other signal quality measurements. The first GNSS positionand first list of beacons are delivered to the processor subsystem andstored locally 203. Since the first GNSS position developed was valid,the GNSS developed information is passed to the location-consumingapplication (not shown). Once the GNSS receiver subsystem has deliveredthe 1^(st) GNSS position, the GNSS receiver system is allowed to enter alow power sleep mode until the next required update 204. The frequencyof required position updates is set by the location-consumingapplication via the processor subsystem. The Beacon Receiver Subsystemalso is allowed to enter a low power sleep mode until the next requiredupdate 205. While the GNSS receiver subsystem is producing valid GNSSposition fixes, the Beacon Receiver Subsystem is activated in parallelto the GNSS receiver subsystem.

When the GNSS receiver subsystem 206 fails to produce a valid GNSSlocation fix 206, in response the Beacon Receiver Subsystemsimultaneously produces a current list of detectable, identifiablebeacons 207. In this case, the controller compares the current beaconlist to the prior beacon list associated in time with the last validGNSS fix.

In the example case, the presence of a limited-range beacon (such as aIEEE-802.11 WiFi node) in both the current and prior beacon lists allowsthe controller to reuse the last valid GNSS position with an increasederror radius. The error radius may be varied according the beacon type(For instance 802.11 versions “b” and “g” access points generally have amaximum range of 100 meters while “n” has an extended range to 200meters. Other WLAN or beacon technologies such as Bluetooth, RFID, andthose using ultra-wideband (UWB) have even shorter ranges in the 10's ofmeters) or frequency band (as the signal frequency increases, the rangedecreased).

If the controller determines that the last valid GNSS fix is still validbased on the examination of the beacon lists, then the controller willswitch the GNSS receiver subsystem off 210 and will set triggeringconditions to reactivate and retry GNSS location at a later time whenthose triggering conditions are met. The beacon receiver subsystem iscommanded to transition to low power state 211 by the controller.

FIG. 2 b depicts the successful resumption of GNSS locations after atriggering event. In this scenario, the GNSS receiver subsystem startsas powered off 210. The controller detects a GNSS retry triggeringevent. These GNSS retry triggering events are detailed in a latersection of this document. In a prime example of a GNSS retry event, asshown in FIG. 2 b, the beacon receiver subsystem produces a fresh beaconlist 211 which is transmitted to the controller. The controller detectsthat the fresh beacon list is missing the previously selected proxybeacon. Since the proxy beacon or beacons selected is no longeracceptable to assure continued location validity, a GNSS reactivationand retry is warranted 212.

The GNSS reactivation and retry 213 is nominally a hot start with thecontroller uploading the current time, last valid location as well asthe stored Almanac and Ephemeris data to the GNSS receiver subsystem. Inthis example, after the hot-start, the GNS receiver successfullyperforms positioning 201 and passes the GNSS generated information tothe controller where it is stored and passed to the application 214. Thecontroller commands the beacon receiver subsystem to rescan for beacons213. The beacon receiver subsystem scans for detectable, identifiablebeacons and reports the determined list to the controller before goinginto low power sleep mode 211. The controller stores the refreshedbeacon list and timestamp 216.

FIG. 2 c depicts the unsuccessful resumption of GNSS locations after atriggering event. In this scenario, the GNSS receiver subsystem startsas powered off 210. The controller detects a GNSS retry triggeringevent. In the example case, shown in FIG. 2 c, the beacon receiversubsystem produces a fresh beacon list 211 which is transmitted to thecontroller. The lack of the previously determined proxy beacon in thefresh beacon list triggers the controller to retry the GNSS subsystem212.

The GNSS reactivation and retry 213 is nominally a hot start with thecontroller uploading the current time, last valid location as well asthe stored Almanac and Ephemeris data to the GNSS receiver subsystem. Inthis example, after the hot-start, the GNS receiver fails to produce aGNSS position fix 215. This failure to produce a new GNSS location ispassed to the controller which then signals the application that currentlocation is unknown or unreliable 216.

FIG. 3 depicts the location device platform (LDP) functional elements.The GNSS receiver Subsystem (GRS) 301 is a low-power commercial GPSchipset connected to the microprocessor-based controller 302 via a databus 308.

The beacon receiver subsystem (BRS) 304 is a low-power commercial WLANor Wireless receiver connected to the microprocessor-based controller302 via a data bus 308 such as a serial peripheral interface (SPI).Functionally, the GRS 301 and BRS 304 are distinct receivers, but mayshare antenna and circuitry.

The Clock 305 is a commercially available clock module. Depending on thedesigner's selection of triggering events, the unit cost, and powerconsumption, the clock can range from a highly stable clock such as aTemperature Compensated Crystal Oscillator to a less stable butinexpensive quartz crystal based real time clock. The Clock 305 will beset and disciplined by the controller using the GNSS derived timingsignal from the GRS 301 when available. The Clock 305 may also be usedto determine low power sleep periods for the GRS 301, the BRS 304 andthe controller 302. If the clock module 305 has been so selected, itwould be possible to provide a reference GNSS system time to the GRS 301allowing a faster time-to-fix.

The controller 302 is a low-power commercial microprocessor whichhandles data transfers between subsystems as well as providing acomputation platform to the decision logic of the LDP. The controller302 may be alternately be implemented as a software program running on aprocessor supplied as part of the ancillary subsystem 306.

The local memory 303 is used to store Almanac and Ephemeris data fromthe GRS 301 as well as beacon information determined by the BRS 304.

The ancillary Subsystems 306 are included to represent the greaterdevice of which the LDP is a part. The Ancillary Subsystems 306 caninclude microprocessors, volatile and non-volatile memory, softwareapplications, a battery monitor, displays and inputs that use ormanipulate the supplied location information.

Broadcasts from Wide Area Communications Networks such as Cellular phonesystems are well suited to serve as proxy beacons. Very small basestations (Femto-cells) resemble WLAN nodes in the range and power.Larger, wider area base stations transmit at high power to allow servicein buildings. Cellular broadcasts are standardized, in specific bands,using specific bandwidths and modulations. Cellular broadcasts containidentifiers (Cell Global Identity (GSM), Cell Identity (UMTS) and BaseID(CDMA2000) allowing for discrimination of transmissions from cells orsectors of a cell. Use of relative power and relative time difference ofarrival techniques allow for detection of movement or changes inposition without need for knowledge of the cellular transmitter'slocation or power.

FIG. 4 depicts the use of beacon signals provided by a wide areacellular network as a proxy for a GNSS developed location estimate. Themobile device 401 is first located at a point 402 outside the building403. The broadcast radio signals 414 from the GNSS constellation 413 canbe successfully used at the first point 402 to generate a validlocation, velocity, altitude and time-of-day estimate. Broadcast signalsfrom the cellular system 405 406 407 408 are also available at the firstpoint 402. At some time, the mobile device 401 moves 415 into thebuilding 403 to the second point 404. The broadcast signals 414 from theGNSS constellation 413 are now blocked by the surrounding structure 403.The high power broadcast signals 409 410 411 412 from the cellularnetwork 405 406 407 408 can successfully penetrate the building allowingthe mobile device 401 to use the broadcast signals 409 410 411 412 as aproxy.

The proxy in this example is dependent on the relative time differencesbetween broadcast signals 409 410 411 412. Since the locations of thecellular network transmitters are not known, a location cannot becalculated from the relative timing differences, but the relative timingdifferences between those received at the first position 402 and thesecond position 404 can be used to determine the approximate change inposition and therefore if the valid GNSS position fix can still beconsidered valid.

FIG. 5 depict the use of beacon signals provided by a wire area cellularnetwork as a proxy for a GNSS developed location estimate. The mobiledevice 501 is first located at a point 502 outside the building 505. Thebroadcast radio signals 520 from the GNSS constellation 519 can besuccessfully used at the first point 502 to generate a valid location,velocity, altitude and time-of-day estimate. Broadcast signals from thecellular system 506 507 508 509 are also available at the first point502. At some time, the mobile device 501 moves 503 into the building 505to the second point 504. The broadcast signals 515 from the GNSSconstellation 514 are now blocked by the surrounding structure 505. Thehigher power broadcast signals 510 511 512 513 from the cellular network506 507 508 4509 can successfully penetrate the building allowing themobile device 401 to use the broadcast signals 510 511 512 513 as aproxy.

The proxy in this example is dependent on the relative power differencesbetween broadcast signals 510 511 512 513. Since the location andtransmission power of the cellular network transmitters are not known, alocation cannot be calculated from the power differences, but therelative power differences between those received at the first position502 and the second position 504 can be used to determine the approximatechange in position and therefore if the valid GNSS position fix from thefirst position 502 can still be considered valid. These relative powerdifferences are shown as the series of hyperbolas 515 516 517 518.

B. GNSS Retry Triggering Events: Loss of Proxy Beacon Trigger

Using the broadcast signal as a proxy for the last known valid GNSSposition, if the proxy can no longer be detected, a GNSS retry isinitiated.

Impending Loss of Hot Start Trigger

The GNSS receiver (in this example, a GPS receiver) collects Almanac andEphemeris data from the satellite broadcasts. The GPS Almanac containsthe orbital parameters for all active GPS satellites and is broadcast byall satellites. The need for precise GPS Almanac data is not stringent,so GPS Almanac data can be considered valid for a several month period.

The Ephemeris data, the very precise orbital and clock correctionbroadcast by each GPS satellite every 30 seconds, is normally collectedfrom each individual satellite broadcast. Ephemeris data is timesensitive and must be refreshed at frequent intervals (for example ofless than 30 minutes) or accurate location estimation will be precluded.

To prevent a ‘cold start’ condition, where the GNSS receiver mustcollect fresh Almanac and Ephemeris or a ‘warm start’ where the GNSSreceiver must collect fresh Ephemeris data before calculating a locationestimation, the LDP will periodically activate the GNSS receiversubsystem to attempt a new GNSS location estimate.

Satellite Freshness Trigger

When Ephemeris data is collected from each available satellite, atimestamp is applied. A database entry of time, satellite ID, andEphemeris data is created within the LDP. As the Ephemeris data agespast the limit (nominally 30 minutes), data for that satellite is markedstale. The trigger for a new GNSS location is set to attempt a satellitesignal collection and location estimation before a database entry ismarked stale. This triggering method relies on a scheme where afterinitial cold-start, satellite ephemeris data is collected by the GNSSreceiver only for newly available satellites or when a guard time(nominally 15 minutes) for reach database entry is exceeded.

Predictive models of satellite availability over time, based on theAlmanac data may also be used to invalidate a database ephemeris entryas the satellite moves over the horizon. This predicted behavior cantrigger a location before the satellite is predicted to unavailable.

One-Shot Timer Exceeded Trigger

The GNSS proxy location validity technique can support an arbitraryvalidity timer. Even if the proxy has not been lost, a GNSS positioningretry will be attempted. This is a resettable timer.

Progressive GPS Retry (Timed) Trigger

The GNSS proxy location validity technique can support an arbitrary orhot-start validity timer. Even if the proxy has not been lost, a GNSSpositioning retry will be attempted. On each failure, the intervalbetween retries will be reduced.

Drift Trigger

A GNSS retry can be required if the proxy beacon power changes overthreshold (up/down) or due to changes in the received relative beaconspower or signal timing.

Neighbors Trigger

A GNSS retry can be required if the proxy beacon list changes. Thistechnique is especially useful in the use of a WLAN node as a proxybeacon.

Last Gasp Trigger

A GNSS retry can be required if the device battery monitor signals thatpower is running low. This trigger is especially useful inlocation-aware tags with persistent, non-volatile memory.

C. Alternative Embodiment—LDP Devices with Communications Transceivers

Registration Retries

When the LDP is equipped with a Wireless Communications Transceiver inplace of the Beacon receiver subsystem, the proxy technique fordetermination of extended GNSS validity can be performed using WirelessCommunications Network (WCN) information other than the Beacon identity,timing and received power. Specifically, data such as received signalstrength (RSS), Timing Advance (TA), Round-Trip-Time (RTT) orRound-Trip-Delay (RTD) become available to the LDP with registered withthe WCN. Since the LDP is not a conforming cellular device, the LDP canretry connecting with cellular network to gather timing and signal datafrom multiple beacons.

Active Determination of Beacon Timing

Since the LDP with a Wireless Communications Transceiver has the abilityto create an active session with the Wireless Communications Network(WCN). The active session need not create a data path through the WCN,requiring the LDP to have a subscription to the WCN provided service anda home server with which to correspond. Rather, the LDP can use a failedsession initiation to receive information from the WCN not available onthe broadcast beacon.

This additional information can include the timing advance (TA) asestimated from the round trip time or the round trip time (RTT) itself,a received signal strength indicator (RSSI), details on the frequenciesor identifiers of neighboring beacons. While the timing and signal powerinformation can be used directly to determine motion, use of theneighboring beacon information can be used to retune the LDP proxyreceiver to develop timing and/or power measurements for each of thebeacons. The added timing and power information would also be used todetect motion which would invalidate the last GNSS obtained position.

D. CONCLUSION

The true scope the present invention is not limited to the presentlypreferred embodiments disclosed herein. In many cases, the place ofimplementation (i.e., the functional element) described herein is merelya designer's preference and not a hard requirement. In addition,modifications and variations of the illustrative embodiments describedabove may be apparent to those skilled in the art after reading thisspecification. Accordingly, except as they may be expressly so limited,the scope of protection of the following claims is not intended to belimited to the specific embodiments described above.

1. A mobile device, comprising: a global navigation satellite system(GNSS) receiver configured to receive GNSS signals; a second receiverconfigured to receive a terrestrial beacon signal; and a controlleroperatively coupled to the GNSS receiver and the second receiver;wherein the mobile device is configured to detect when GNSS signals areunavailable due to signal blockage and, in response thereto, to use areceived terrestrial beacon signal to determine whether a previous GNSSlocation update is valid; and wherein, in determining whether a previousGNSS location update is valid, the mobile device is configured tocompare a current beacon list to a prior beacon list associated in timewith a last valid GNSS fix.
 2. A mobile device as recited in claim 1,wherein the controller is further configured to employ the secondreceiver to acquire network parameters from the terrestrial beacon andto determine if a last known GNSS-derived position is still valid.
 3. Amobile device as recited in claim 2, wherein said network parametersinclude at least one of: beacon power, beacon identifier, timingadvance, and round-trip-time.
 4. A mobile device as recited in claim 1,wherein the controller is further configured to suspend GNSS searchesand to power down the GNSS receiver in response to detecting that GNSSsignals are unavailable.
 5. A mobile device as recited in claim 1,wherein the mobile device is integrated into a mobile phone.
 6. A mobiledevice as recited in claim 1, wherein the mobile device is integratedinto a personal or automobile navigation system.
 7. A mobile device asrecited in claim 1, wherein the mobile device is integrated into a tagdevice.
 8. A mobile device as recited in claim 1, wherein the mobiledevice is battery powered.
 9. A mobile device as recited in claim 1,wherein the GNSS receiver and second receiver are implemented in asoftware defined radio (SDR).
 10. A mobile device as recited in claim 1,wherein the mobile device is configured to receive data via receivedbeacon signals, wherein the received data includes identifyinginformation for a beacon and its network.
 11. A mobile device as recitedin claim 10, wherein the mobile device is further configured todetermine network timing from frames transmitted by the beacon.
 12. Amobile device as recited in claim 11, wherein the mobile device isfurther configured to receive at least one of a pilot channel signal anda synchronization channel signal from the beacon.
 13. A mobile device asrecited in claim 11, wherein the mobile device is further configured toreceive beacon transmissions including at least one of a broadcastcontrol channel of a cellular network and a beacon frame of an IEEE802.11 network.
 14. A mobile device as recited in claim 11, wherein themobile device is further configured to receive beacon identifiersincluding at least one of a cell-ID in a CDMA or UMTS cellular network,a cell-global-identifier (CGI) in a GSM cellular network, and a basestation identifier in an IEEE 802.11 wireless local area network.
 15. Amobile device as recited in claim 1, wherein the mobile device isfurther configured to passively use received beacon signals frommultiple beacons as a proxy for a high accuracy GNSS location estimateby measuring and comparing the strengths of signals received fromneighboring beacons during periodic sampling events.
 16. A mobile deviceas recited in claim 1, wherein the mobile device is further configuredto perform a process including the following steps, whereby the devicedetermines whether a previous GNSS location update is valid: the GNSSreceiver is employed to develop a first valid GNSS position includinglatitude, longitude, altitude, and timing; in parallel, the secondreceiver is employed to detect and identify one or more radio beacons inpredefined transmission bands; the first valid GNSS position and a firstlist of beacons are stored locally within the mobile device; the GNSSposition information is passed to a location-consuming application; theGNSS receiver enters a low power sleep mode until a next required updatetime as determined by the location-consuming application; the secondreceiver enters a low power sleep mode until the next required updatetime; upon determining that the GNSS receiver has failed to produce avalid GNSS location fix, the second receiver produces a current list ofdetectable, identifiable beacons; the current beacon list is compared tothe prior beacon list associated in time with the last valid GNSS fix;and upon determining the presence of a limited-range beacon in both thecurrent and prior beacon lists, the mobile device reuses the last validGNSS position, and then switches the GNSS receiver off and setstriggering conditions to reactivate and retry GNSS location at a latertime when those triggering conditions are met.
 17. A mobile device asrecited in claim 16, wherein the mobile device is further configured toperform a process including the following steps, whereby the deviceresumes GNSS locations after a triggering event: a GNSS retry triggeringevent is detected; the controller uploads the current time, last validlocation, and stored almanac and ephemeris data to the GNSS receiver;the GNS receiver successfully performs positioning and GNSS generatedinformation is stored and passed to the location-consuming application;the second receiver scans for detectable, identifiable beacons andreports a list to the controller and then goes into the low power sleepmode; and the refreshed beacon list and a timestamp are stored.
 18. Amobile device as recited in claim 1, wherein the mobile device isfurther configured to use both the GNSS receiver and the second receiverto collect information from each receiver concurrently, and to make ameasurement of the beacon's transmit power and to compare themeasurement to the beacon's transmit power recorded at a previous GNSSlocation fix, and based on the comparison to detect motion toward oraway from the beacon.
 19. A mobile device as recited in claim 1, whereinthe mobile device is further configured to perform a process includingthe following steps, whereby the device determines whether a previousGNSS location update is valid: the GNSS receiver is employed to developa first valid GNSS position including latitude, longitude, altitude, andtiming; in parallel, the second receiver is employed to detect andidentify one or more radio beacons in predefined transmission bands,wherein the radio beacons comprise broadcast signals from cellularnetwork transmitters; the GNSS position information is passed to alocation-consuming application; the GNSS receiver enters a low powersleep mode until a next required update time as determined by thelocation-consuming application; the second receiver enters a low powersleep mode until the next required update time; upon determining thatthe GNSS receiver has failed to produce a valid GNSS location fix, thesecond receiver produces a current list of detectable, identifiablebeacons; the current beacon list is compared to the prior beacon listassociated in time with the last valid GNSS fix, and one of relativetiming and power differences between beacon signals are used todetermine an approximate change in position; and based on theapproximate change in position, the mobile device determines thevalidity of last valid GNSS position.
 20. A mobile device as recited inclaim 1, wherein the second receiver is implemented in a communicationstransceiver.
 21. A mobile device as recited in claim 20, wherein themobile device is configured to employ the communications transceiver toretry connecting with a cellular network to gather timing and signaldata from multiple beacons.
 22. A mobile device as recited in claim 20,wherein the mobile device is configured to employ the communicationstransceiver to create an active session with a cellular network and touse a failed session initiation to receive additional information fromthe network that is unavailable on a broadcast beacon.
 23. A mobiledevice as recited in claim 22, wherein the additional informationcomprises at least one of timing advance (TA), a received signalstrength indicator (RSSI), and details on frequencies or identifiers ofneighboring beacons.
 24. A method for use in determining the location ofa mobile device, comprising: employing a global navigation satellitesystem (GNSS) receiver associated with the mobile device to receiveglobal navigation satellite system (GNSS) signals; employing a secondreceiver associated with the mobile device to receive a terrestrialbeacon signal; and employing a controller associated with the mobiledevice to detect when GNSS signals are unavailable due to signalblockage and, in response thereto, to use a received terrestrial beaconsignal to determine whether a previous GNSS location update is valid;and wherein, in determining whether a previous GNSS location update isvalid, a current beacon list is compared to a prior beacon listassociated in time with a last valid GNSS fix.
 25. A method as recitedin claim 24, further comprising employing the second receiver to acquirenetwork parameters from the terrestrial beacon and determining if a lastknown GNSS-derived position is still valid.
 26. A method as recited inclaim 25, wherein said network parameters include at least one of:beacon power, beacon identifier, timing advance, and round-trip-time.27. A method as recited in claim 24, further comprising suspending GNSSsearches and powering down the GNSS receiver in response to detectingthat GNSS signals are unavailable.
 28. A method as recited in claim 24,wherein the mobile device is integrated into a mobile phone.
 29. Amethod as recited in claim 24, wherein the mobile device is integratedinto a personal or automobile navigation system.
 30. A method as recitedin claim 24, wherein the mobile device is integrated into a tag device.31. A method as recited in claim 24, wherein the mobile device isbattery powered.
 32. A method as recited in claim 24, wherein the GNSSreceiver and second receiver are implemented in a software defined radio(SDR).
 33. A method as recited in claim 24, wherein the method furthercomprises receiving data via received beacon signals, wherein thereceived data includes identifying information for a beacon and itsnetwork.
 34. A method as recited in claim 33, wherein the method furthercomprises determining network timing from frames transmitted by thebeacon.
 35. A method as recited in claim 34, wherein the method furthercomprises receiving at least one of a pilot channel signal and asynchronization channel signal from the beacon.
 36. A method as recitedin claim 34, wherein the method further comprises receiving beacontransmissions including at least one of a broadcast control channel of acellular network and a beacon frame of an IEEE 802.11 network.
 37. Amethod as recited in claim 34, wherein the method further comprisesreceiving beacon identifiers including at least one of a cell-ID in aCDMA or UMTS cellular network, a cell-global-identifier (CGI) in a GSMcellular network, and a base station identifier in an IEEE 802.11wireless local area network.
 38. A method as recited in claim 24,wherein the method further comprises passively using received beaconsignals from multiple beacons as a proxy for a high accuracy GNSSlocation estimate by measuring and comparing the strengths of signalsreceived from neighboring beacons during periodic sampling events.
 39. Amethod as recited in claim 24, wherein the method further comprisesperforming a process including the following steps, whereby the methoddetermines whether a previous GNSS location update is valid: the GNSSreceiver is employed to develop a first valid GNSS position includinglatitude, longitude, altitude, and timing; in parallel, the secondreceiver is employed to detect and identify one or more radio beacons inpredefined transmission bands; the first valid GNSS position and a firstlist of beacons are stored locally within the mobile device; the GNSSposition information is passed to a location-consuming application; theGNSS receiver enters a low power sleep mode until a next required updatetime as determined by the location-consuming application; the secondreceiver enters a low power sleep mode until the next required updatetime; upon determining that the GNSS receiver has failed to produce avalid GNSS location fix, the second receiver produces a current list ofdetectable, identifiable beacons; the current beacon list is compared tothe prior beacon list associated in time with the last valid GNSS fix;and upon determining the presence of a limited-range beacon in both thecurrent and prior beacon lists, the mobile device reuses the last validGNSS position, and then switches the GNSS receiver off and setstriggering conditions to reactivate and retry GNSS location at a latertime when those triggering conditions are met.
 40. A method as recitedin claim 39, wherein the method further comprises performing a processincluding the following steps, whereby the device resumes GNSS locationsafter a triggering event: a GNSS retry triggering event is detected; thecontroller uploads the current time, last valid location, and storedalmanac and ephemeris data to the GNSS receiver; the GNS receiversuccessfully performs positioning and GNSS generated information isstored and passed to the location-consuming application; the secondreceiver scans for detectable, identifiable beacons and reports a listto the controller and then goes into the low power sleep mode; and therefreshed beacon list and a timestamp are stored.
 41. A method asrecited in claim 24, wherein the method further comprises using both theGNSS receiver and the second receiver to collect information from eachreceiver concurrently, and to make a measurement of the beacon'stransmit power and to compare the measurement to the beacon's transmitpower recorded at a previous GNSS location fix, and based on thecomparison to detect motion toward or away from the beacon.
 42. A methodas recited in claim 24, wherein the method further comprises performinga process including the following steps, whereby the device determineswhether a previous GNSS location update is valid:
 43. A method asrecited in claim 24, wherein the second receiver is implemented in acommunications transceiver.
 44. A method as recited in claim 43, whereinthe method further comprises employing the communications transceiver toretry connecting with a cellular network to gather timing and signaldata from multiple beacons.
 45. A method as recited in claim 43, whereinthe method further comprises employing the communications transceiver tocreate an active session with a cellular network and to use a failedsession initiation to receive additional information from the networkthat is unavailable on a broadcast beacon.
 46. A method as recited inclaim 45, wherein the additional information comprises at least one oftiming advance (TA), a received signal strength indicator (RSSI), anddetails on frequencies or identifiers of neighboring beacons.
 47. Asystem for use in determining the location of a mobile device,comprising: means for employing a global navigation satellite system(GNSS) receiver associated with the mobile device to receive globalnavigation satellite system (GNSS) signals; means for employing a secondreceiver associated with the mobile device to receive a terrestrialbeacon signal; and means for employing a controller associated with themobile device to detect when GNSS signals are unavailable due to signalblockage and, in response thereto, to use a received terrestrial beaconsignal to determine whether a previous GNSS location update is valid.48. A system as recited in claim 47, further comprising means for meansfor employing the second receiver to acquire network parameters from theterrestrial beacon and determining if a last known GNSS-derived positionis still valid.
 49. A system as recited in claim 48, wherein saidnetwork parameters include at least one of: beacon power, beaconidentifier, timing advance, and round-trip-time.
 50. A system as recitedin claim 47, further comprising means for suspending GNSS searches andpowering down the GNSS receiver in response to detecting that GNSSsignals are unavailable.
 51. A system as recited in claim 47, whereinthe mobile device is integrated into one of a mobile phone, a personalor automobile navigation system, and a tag device; and wherein themobile device is battery powered.
 52. A system as recited in claim 47,wherein the GNSS receiver and second receiver are implemented in asoftware defined radio (SDR).
 53. A system as recited in claim 47,wherein the system further comprises means for receiving data viareceived beacon signals, wherein the received data includes identifyinginformation for a beacon and its network.
 54. A system as recited inclaim 53, wherein the system further comprises means for determiningnetwork timing from frames transmitted by the beacon.
 55. A system asrecited in claim 54, wherein the system further comprises means forreceiving at least one of a pilot channel signal and a synchronizationchannel signal from the beacon.
 56. A system as recited in claim 54,wherein the system further comprises means for receiving beacontransmissions including at least one of a broadcast control channel of acellular network and a beacon frame of an IEEE 802.11 network.
 57. Asystem as recited in claim 54, wherein the system further comprisesmeans for receiving beacon identifiers including at least one of acell-ID in a CDMA or UMTS cellular network, a cell-global-identifier(CGI) in a GSM cellular network, and a base station identifier in anIEEE 802.11 wireless local area network.
 58. A system as recited inclaim 47, wherein the system further comprises means for passively usingreceived beacon signals from multiple beacons as a proxy for a highaccuracy GNSS location estimate by measuring and comparing the strengthsof signals received from neighboring beacons during periodic samplingevents.
 59. A system as recited in claim 47, wherein the system furthercomprises means for performing a process including the following steps,whereby the system determines whether a previous GNSS location update isvalid: the GNSS receiver is employed to develop a first valid GNSSposition including latitude, longitude, altitude, and timing; inparallel, the second receiver is employed to detect and identify one ormore radio beacons in predefined transmission bands; the first validGNSS position and a first list of beacons are stored locally within themobile device; the GNSS position information is passed to alocation-consuming application; the GNSS receiver enters a low powersleep mode until a next required update time as determined by thelocation-consuming application; the second receiver enters a low powersleep mode until the next required update time; upon determining thatthe GNSS receiver has failed to produce a valid GNSS location fix, thesecond receiver produces a current list of detectable, identifiablebeacons; the current beacon list is compared to the prior beacon listassociated in time with the last valid GNSS fix; and upon determiningthe presence of a limited-range beacon in both the current and priorbeacon lists, the mobile device reuses the last valid GNSS position, andthen switches the GNSS receiver off and sets triggering conditions toreactivate and retry GNSS location at a later time when those triggeringconditions are met.
 60. A system as recited in claim 59, wherein thesystem further comprises means for performing a process including thefollowing steps, whereby the device resumes GNSS locations after atriggering event: a GNSS retry triggering event is detected; thecontroller uploads the current time, last valid location, and storedalmanac and ephemeris data to the GNSS receiver; the GNS receiversuccessfully performs positioning and GNSS generated information isstored and passed to the location-consuming application; the secondreceiver scans for detectable, identifiable beacons and reports a listto the controller and then goes into the low power sleep mode; and therefreshed beacon list and a timestamp are stored.
 61. A system asrecited in claim 47, wherein the system further comprises means forusing both the GNSS receiver and the second receiver to collectinformation from each receiver concurrently, and to make a measurementof the beacon's transmit power and to compare the measurement to thebeacon's transmit power recorded at a previous GNSS location fix, andbased on the comparison to detect motion toward or away from the beacon.62. A system as recited in claim 47, wherein the system furthercomprises means for performing a process including the following steps,whereby the device determines whether a previous GNSS location update isvalid: the GNSS receiver is employed to develop a first valid GNSSposition including latitude, longitude, altitude, and timing; inparallel, the second receiver is employed to detect and identify one ormore radio beacons in predefined transmission bands, wherein the radiobeacons comprise broadcast signals from cellular network transmitters;the GNSS position information is passed to a location-consumingapplication; the GNSS receiver enters a low power sleep mode until anext required update time as determined by the location-consumingapplication; the second receiver enters a low power sleep mode until thenext required update time; upon determining that the GNSS receiver hasfailed to produce a valid GNSS location fix, the second receiverproduces a current list of detectable, identifiable beacons; the currentbeacon list is compared to the prior beacon list associated in time withthe last valid GNSS fix, and one of relative timing and powerdifferences between beacon signals are used to determine an approximatechange in position; and based on the approximate change in position, themobile device determines the validity of last valid GNSS position.
 63. Asystem as recited in claim 47, wherein the second receiver isimplemented in a communications transceiver.
 64. A system as recited inclaim 63, wherein the system further comprises means for employing thecommunications transceiver to retry connecting with a cellular networkto gather timing and signal data from multiple beacons.
 65. A system asrecited in claim 63, wherein the system further comprises means foremploying the communications transceiver to create an active sessionwith a cellular network and to use a failed session initiation toreceive additional information from the network that is unavailable on abroadcast beacon.
 66. A system as recited in claim 65, wherein theadditional information comprises at least one of timing advance (TA), areceived signal strength indicator (RSSI), and details on frequencies oridentifiers of neighboring beacons.